Accurate measurements of cluster masses are crucial for probing growth of structure, and consequently, for constraining dark energy parameters and theories of modified gravity. The gravitational lensing effect of a cluster is sensitive to its total mass (including dark matter). In particular, relic radiation from the recombination era (CMB) acts as a backlight that is lensed by massive clusters. This provides a method of constraining masses complementary to optical weak lensing, with the advantage that the source plane has a very well determined redshift and statistical properties. I will report on the first detection of this effect. The lensing field was reconstructed from CMB temperature observations using the ACTPol telescope and stacked at the location of CMASS galaxies which trace dark matter halos of ~10^13 M_solar galaxy groups, providing a 3.2 sigma detection and a ~35% mass constraint. I will also briefly touch on the capabilities of future CMB experiments to use this method to constrain dark energy parameters.